1. Field of the Invention
The present invention relates to the fabrication of semiconductor devices having feature sizes in the range of 90 nm and smaller. In particular, the invention relates to a method of maintaining the adhesion of a photoresist to a surface during development of a pattern in the photoresist and to maintenance of the functionality of a chemically amplified photoresist on the surface of a dielectric anti-reflection coating (DARC).
2. Description of the Background Art
As semiconductor devices are becoming ever smaller, the device features necessarily become smaller. To produce feature sizes in the range of about 124 nm, for example, a chemically amplified photoresist (CAR) is pattern imaged using a DUV wavelength in the range of about 248 nm. To produce the next generation of feature sizes, in the range of 90 nm, the CAR will be pattern imaged using a radiation wavelength in the range of about 193-198 nm. The chemically amplified photoresists are typically deposited over the surface of a DARC which reduces reflection during pattern radiation imaging of the CAR. The composition of the DARC is determined by the refractive index and extinction coefficient required to attenuate the radiation reflected off the surface of the device substrate which underlies the DARC.
The DARC used in combination with the CAR deep UV (DUV) photoresists are frequently deposited by plasma enhanced chemical vapor deposition (PECVD). A number of techniques have been described for deposition of PECVD films. In general, the deposition techniques are closely tied to the apparatus used for the deposition, although some parameters such as process chamber pressure, substrate temperature and composition of the source gas used to provide the reactive species are relatively independent of the apparatus peculiarities.
In an article entitled “Dual Microwave—R. F. Plasma Deposition Of Functional Coatings” by J. E. Klemberg-Sapieha et al. in Thin Solid Films, 193/194 (1990) 965-972, the authors described the deposition of plasma silicon nitride (P—SiN) and amorphous hydrogenated silicon (a-Si:H) films using a dual-frequency plasma. The power source for the plasma consisted of a microwave discharge with RF power simultaneously superimposed on the substrate holder. The negative substrate bias voltage was said to substantially affect the deposition rate, the film composition, and the film electrical properties. The authors report that ionic species are estimated to contribute about 30% to 40% to the film growth rate. The increasing ion flux and energy with increasing substrate bias voltage is said to enhance the formation of densely packed coatings. As a result, the dielectric los tan δ of P—SiN, and the resistivity of a-Si:H is said to be reduced by several orders of magnitude when the substrate bias voltage is raised from 0 to −800V. The depositions were carried out in a large volume microwave plasma (LMP®) apparatus of the kind available from AIXTRON AG and Fraunhofer IAF, Freiburg, Germany, with a MW power at 2.45 GHz applied through a fused silica window from a periodic slow wave structure. The substrate bias was applied at a frequency of 13.56 MHz to a powered electrode which functioned as the substrate holder.
A. Raveh et al. discuss the “Deposition and properties of diamond like carbon films produced in microwave and radio-frequency plasma” in an article in J. Vac. Sci. Technol. A 10(4), July/August 1992. In that article the authors report that hard a-C:H films were grown in a dual frequency plasma sustained simultaneously by microwave and radio-frequency power. Optimum growth conditions, namely those leading to the most pronounced sp3 structural features in the films, are said to depend very strongly on the methane feed gas flow rate and on the argon concentration in the case of CH4/Ar feed gas mixtures. The optimum conditions are reported to be found to correspond to the maximum values of ion flux at the growing film surface in combination with high concentrations of precursor species such as CH, C2, C3, and atomic hydrogen in the plasma, as revealed by optical emission spectroscopy. Films grown under optimum conditions are said to have very high microhardness (˜50 GPa), high density (1.8 g/cm3), and low internal stress (0.5 GPa). Addition of argon to the methane in the feed gas is indicated as enhancing the gas phase fragmentation and raising microhardness, but argon atoms trapped in the film structure increased internal stress. The apparatus which was used to produce the films was the same apparatus as described above.
International Application No. PCT/US00/20383, of Gill Yong Lee, published Feb. 8, 2001, discloses the use of a silicon-rich layer over the surface of a dielectric ARC to prevent “resist poisoning”. In particular, the DARC described is an inorganic ARC layer such as silicon nitride (SixNy) or silicon oxynitride (SiNxOy), or hydrogenated silicon oxynitride. The DARC is said to be particularly useful during pattern imaging of the photoresist, typically a CAR which relies on an acid formed in irradiated areas to enable development of the pattern. However, the presence of amine radicles which are contributed by the DARC contaminates the CAR applied over the DARC, neutralizing the acid-generators. This makes the contaminated portions of the resist insoluble by the developer. As a result, a “foot” is present at the base of the developed resist profile. To prevent this problem, a capping layer is applied over the DARC prior to application of the CAR. In one embodiment, the capping layer is silicon, preferably a thin amorphous silicon layer. The silicon layer is said to be sufficiently thin to avoid causing standing waves and interference in the resist. Alternatively, the cap layer may be a mono-atomic layer that alters the surface morphology of the DARC. The mono-atomic layer, in one embodiment comprises excess silicon dangling bonds on the surface of the DARC. For example, the cap layer could comprise a silicon-rich oxide or a silicon-rich oxynitride if the DARC comprised silicon oxide or an oxynitride layer.
U.S. Pat. No. 6,227,141 of Sharan et al., issued May 8, 2001, describes an RF powered, plasma enhanced chemical vapor deposition reactor and methods of use of the reactor. The plasma enhanced chemical vapor deposition (PECVD) apparatus makes use of a first RF power source which delivers RF power at a first frequency to a first electrode, and a second RF power source which delivers RF power at a second frequency to a second electrode.
Applied Materials, Inc., Santa Clara, Calif. offers both single and dual frequency PECVD chambers. The multifrequency processing chambers typically apply two different RF power frequencies to a single electrode.
U.S. Pat. No. 6,171,764 to Ku et al., issued Jan. 9, 2001 describes the kinds of radiation reflection problems which may occur in photolithographic processes. The description relates to semiconductor manufacturing processes which make use of a dielectric anti-reflective (DARC) layer to reduce reflected radiation during photoresist imaging. In particular, the difference between the Ku et al. invention and other known methods is based on the ordering of specific layers in the substrate used in the photolithographic process. In the Ku et al. method, the DARC layer is applied over a substrate, followed by a hard mask layer, and then a photoresist. This is said to compare with other known methods where the DARC layer is used between the photoresist layer and the hard mask layer. (Col. 3, lines 35-46.)
U.S. Pat. No. 6,607,984 to Lee et al., issued Aug. 19, 2003 describes a method of semiconductor fabrication in which an inorganic anti-reflection coating is employed and subsequently removed by selective etching relative to an underlying inorganic dielectric layer. (Col. 1, lines 61-67, continuing at Col. 2 lines 1-6.)
European Patent Application No. 99204265.5 of Shao-Wen Hsia et al., published Jun. 21, 2000, describes a semiconductor interconnect structure employing an inorganic dielectric layer produced by plasma enhanced chemical vapor deposition (pecvd). In accordance with a preferred embodiment of the invention, a metal layer upon which photoresist patterns are developed comprises a sandwiched metal stack having a layer of conducting metal (aluminum, titanium, and the like) bounded by an upper thin-film ARC layer and a bottom thin-film barrier layer, where at least the top layer is composed of an inorganic dielectric substance. The use of an inorganic dielectric top ARC layer is said to facilitate the use of thinner photoresist layers while preserving the integrity of the photoresist pattern for deep sub-micron feature sizes. (Col. 1, lines 56-58, continuing at Col. 2, lines 1-8.)
We have encountered a problem which does not appear to be addressed in the known art, but which has become important in particular with respect to semiconductor substrate features in the 90 nm range and smaller. During development of the photoresist, applicants have encountered instances where the photoresist becomes detached from the underlying substrate. Development refers to treatment of the photoresist with a fluid, typically a liquid reagent, to remove portions of the photoresist, thus creating a pattern. For reference purposes, when the portions of the photoresist which are removed are the portions which have been exposed to patterning radiation, the photoresist is said to be a positive photoresist. When the portions of the photoresist which are removed are the portions which have not been exposed to patterning radiation, the photoresist is said to be a negative photoresist.
In addition to detachment of areas of the photoresist from the underlying substrate, we have continued to observe reaction at the interface between an underlying DARC and the photoresist. This reaction is despite the use of a nitrogen-free DARC. It is possible to use a capping layer of the kind described in the art to isolate the photoresist from an underlying DARC. However, typically the semiconductor manufacturing process is a dual damascene process, which is common in multilevel metal devices. In a dual damascene process, after the first photoresist patterning process, there is an etch through underlying layers, including the DARC using the photoresist as a pattern. Subsequently, the portions of the opened pattern are filled with a buried ARC (BARC), followed by application of a second layer of photoresist and creation of a second pattern in the photoresist. Typically the second layer of photoresist is in contact with the DARC at the surface area where the etch passed through the DARC. Thus, there are still significant photoresist “poisoning” problems even when a capping layer is applied over the upper surface of the DARC as a part of the preparation for the first patterning step.
A need exists for ensuring photoresist adhesion and uniform lithographic imaging and development activity on the surface of various underlying substrates (particularly on the surface of dielectric arcs) during the fabrication of semiconductor devices with feature sizes of 90 nm and smaller.